This invention relates to an image tube, particularly to the one equipped with an improved output fluorescent screen.
In general, a fluorescent screen of an image tube is prepared by tightly depositing phosphor particles on a transparent substrate by precipitation, slurry coating, electrodeposition, etc., followed by coating the surface of the resultant phosphor layer with a thin film of metal or the like. The fluorescent screen is deeply related with the properties of the image tube, particularly, with the improvement of brightness, resolution and contrast.
Appended FIG. 1 shows an X-ray image intensifier as an example of the conventional image tube. As shown in the drawing, the intensifier comprises an evacuated envelope 1 made of glass and an input screen 2 disposed on the input side of the envelope 1. The input screen 2 comprises a substrate 3, a phosphor layer 4 and a photocathode 5. The substrate is formed of a curved plate having a predetermined curvature and is mounted in the envelope 1. It is seen that the convex surface of the substrate 3 faces the input side of the envelope. The phosphor layer 4 is made of cesium iodide, which contains sodium as an activator. As shown in the drawing, this layer is formed on the concave surface of the substrate in a thickness of 100 to 250 .mu.. The photocathode 5 is formed on the surface of the phosphor layer 4.
Accelerating electrodes 7 and an output fluorescent screen 8 are mounted on the output side of the envelope 1. Further, focussing electrodes 6 are provided adjacent to the inner wall of the envelope 1.
When an X-ray 9 is incident on the input screen 2, the phosphor layer 4 is caused to fluoresce. Namely, the X-ray 9 is converted to light. Further, the light emitted from the phosphor layer 4 is converted to electron by the photocathode 5, resulting in photoemission from the photocathode. The photoelectron emitted from the photocathode is then accelerated by the accelerating electrodes 7 while being focussed by the focussing electrodes 6, thereby causing the output fluorescent screen 8 to fluoresce. In short, the X-ray image incident on the input screen is reproduced on the output screen as a optical image. In this case, the reproduced optical image is reduced in size to about one-tenth of the original X-ray image. Further, the photoelectron is accelerated by the accelerating electrodes 7. Those combine to render the optical image on the output screen several thousand times as bright as the image on the input screen.
FIG. 2 shows a cross section of the output fluorescent screen of the X-ray image intensifier outlined above. As shown in the drawing, the output screen comprises a transparent substrate 10, a phosphor layer 12 and an aluminum film 13. The phosphor layer 12 is formed by depositing phosphor particles 11 sized at 0.2 to 3.mu. on the transparent substrate 10 in a thickness of about 5 to 15.mu.. In this case, the packing density of the phosphor particles is about 40%. On the other hand, the aluminum film 13 is formed by vacuum deposition. To be more specifically a nitrocellulose thin film is formed first on the surface of the phosphor layer 12, followed by vacuum deposition of aluminum in vacuum of about 10.sup.-5 torr to form an aluminum layer having a thickness of 3,000 to 4,000A. Finally, heating is effected under the air atmosphere so as to remove the nitrocellulose film.
What should be noted is that the aluminum film is formed by depositing aluminum atoms on the nitrocellulose film formed on the surface of the phosphor layer having a packing density of phosphor particles as low as about 40%. Naturally, the aluminum film is rendered rough. In addition, the nitrocellulose film is removed by heating. It follows that the bonding strength is negligible between the aluminum layer and the phosphor particles facing the aluminum layer.
The photoelectron emitted from the input screen 2 (FIG. 1) is accelerated and focussed to pass through the aluminum film 13, thereby causing the phosphor particles 11 to fluoresce and reproducing the X-ray image as an optical image. The optical image thus formed can be observed through the transparent substrate 10 and an output window of the envelope 1.
The following drawbacks are inherent in the conventional output fluorescent screen of the image tube described above:
1. Since the aluminum film 13 is high in reflectance, the light emitted by the phosphor particles 11 is reflected and scattered by the aluminum film 13, resulting in an unsatisfactory contrast and a low resolution.
2. The aluminum film 13 is formed by vapor deposition on a nitrocellulose film formed on the surface of the phosphor layer 12. Since the phosphor layer 12 is porous and has a rough surface, the nitrocellulose film also comes to have a rough surface. Naturally, the aluminum film 13 formed on the nitrocellulose film becomes uneven in thickness with respect to the direction in which the light emitted by the phosphor layer travels. It follows that the light emitted by the phosphor layer partly penetrates the aluminum layer to reach the photocathode of the input screen, resulting in that photoelectron is emitted again from the photocathode of the input screen. This brings about deterioration of contrast and resolution.
3. The problem mentioned in item 2 may be avoided by thickening the aluminum film 13. In this case, however, the aluminum film tends to peel off the phosphor layer 12. In addition, the thickened film obstructs the passage of the photoelectron therethrough, leading to a decreased brightness of the resultant optical image.